1. Field of the Invention
The present invention relates to a controlling apparatus for an electrically-driven power steering system installed on an automobile.
2. Description of the Related Art
An electrically-driven power steering system installed on an automobile is designed to assist an operation force necessary for steering the automobile by using a motor. The system's controlling apparatus has a torque sensor which detects a rotational force applied to a steering wheel and a motor which assists the operation of a steering mechanism are disposed on the steering mechanism to which the steering wheel is connected in an interlocking fashion through a shaft. The operation force of the steering wheel is reduced by driving the motor responsive to detection values of the torque sensor.
In such a power steering system, there were problems of deterioration in effectiveness of the steering mechanism because of a phase delay in an electric circuit of the controlling apparatus due to the motor inertia and friction introduced by a reduction unit interposed between the motor and the steering mechanism. In order to solve the problems, a technique of adjusting a motor current by differentiating detecting signals of the torque sensor is shown in Japanese Patent Application Laid-Open No. 61-115771 (1986). This technique performs a differential control, which operates the steering mechanism without a time delay by differentiating the detecting signal of the torque sensor, adding a differentiated signal to a temporary control value obtained responsive to the detecting signal of the torque sensor, and responding to a steering force detected by the torque sensor.
In an apparatus in which such differential control is implemented, the torque sensor detecting signal and a temporary target value of a motor driving current are functionally processed beforehand. The temporary target value of the motor driving current is decided from the torque sensor detecting signal by using this function, and the differentiated signal of the torque sensor detecting signal is added to this temporary target value to decide a final target value of the motor driving current. Then, the motor current is controlled so as to coincide with the decided target value to assist the steering. In this case, for example, such as disclosed in U.S. patent application No. 686,107 now U.S. Pat. No. 5,201,818, an apparatus which uses a high gain signal is obtained by amplifying the output signal of the torque sensor, or a low gain signal obtained by not amplifying the output signal of the torque sensor, as a detecting signal which is subject to the differential processing.
In a conventional controlling apparatus for a power steering system which performs the differential control there were such problems as described below. In case of using the high gain signal, though its differential amount is large, on the other hand, since this signal has a high gain, it becomes saturated above a predetermined torque. Thus the differential control does not work and an inharmonious feeling, such as a feeling of hooking, is produced in steering feeling because the differential amount becomes zero in the saturated range. Meanwhile, in case of using a low gain signal, though the differential control works in a wide range of steering torque, on the other hand, since the differential amount is small, an effect of differential control is not obtained sufficiently. Thus, when its differential constant is increased to obtain the effect of differential control sufficiently, the differential value is liable to oscillate, thereby producing vibration in steering.